Every walk has become a search. Chicken bones, a wrapper, half a bagel, something unidentifiable in the long grass. Your dog's head goes down and by the time you reach them it is already swallowed.
Why this is so hard to beat
Food on the ground pays immediately, every single time, with no effort. Anything you offer arrives later and depends on your dog choosing you over a thing already in their mouth.
Adolescence adds two problems on top. Your dog now covers ground fast enough to reach things before you do. Dogs also respond less to the cues of the person who raised them around the onset of the stage.
The three things that actually move it
Walk the same routes and learn the hotspots. Behind the bins, the bench, the path from the takeaway. Shorten the leash to 3 feet 20 paces before each one, then let it out again after.
Change the route for 2 weeks if one street is doing most of the damage. Removing rehearsal is faster than out-training it.
Carry something better than litter, and pay generously. Cheese or roast chicken, not the biscuits your dog ignores outdoors.
Train the trade indoors first, 8 minutes
Never chase and never prise something out of your dog's mouth. Both teach a dog to swallow fast and to move away from you when they find something.
Indoors, give your dog a boring item, a rolled sock. Hold 3 pieces of cheese at their nose and say your cue once, in a level voice.
The moment the sock drops, feed all 3 pieces, one at a time, then hand the sock straight back. Getting things back is what stops guarding developing.
Do 6 reps and stop. Repeat for 5 days, then move to the garden with a slightly more interesting item.
Outdoors, use the same cue on something worthless first, a leaf or a stick, so the cue is not only ever used in an emergency.
Reward-based training has been associated with higher reported responsiveness and fewer reported problem behaviours. This is a place where the size of the payment decides whether it works.
When your dog will not swap
Scatter 6 treats on the ground 4 feet away instead. Most dogs leave one item for a spread of many, and you get the item back without a hand near their mouth.
If your dog has started guarding what they pick up, with stiffening or a growl, stop this plan. That needs a qualified behaviour professional.
Safety note
Some things are a medical emergency, not a training problem. Call your veterinarian straight away for cooked bones, corn cobs, rotting meat, mushrooms, rodent bait, human medication, cannabis, chocolate, grapes or raisins. Call straight away for anything at all if your dog then vomits repeatedly, strains, or goes quiet and flat. Sudden behaviour change warrants a veterinary examination.
A basket muzzle is not a punishment
For a dog who eats genuinely dangerous things, consider a well-fitted basket muzzle introduced with food over 2 weeks. It lets them sniff, pant, drink and take treats while keeping litter out. It is management, and management is half of behaviour change.
Today's version, in one line: 6 paid swaps indoors with a rolled sock, and cheese in your pocket tomorrow.